An IPS-type liquid crystal cell is configured to, under an electric field-off state, achieve a homogeneous orientation or alignment of liquid crystal molecules in which liquid crystal molecules are uniformly oriented or aligned in one direction in a plane of the liquid crystal cell. In a liquid crystal display device equipped with such an IPS-type liquid crystal cell having a structure as described above, a pair of polarizers are disposed, respectively, on opposite sides of the liquid crystal cell in such a manner that absorption axes of the polarizers orthogonally intersect each other. Further, one of the polarizers is disposed with respect to the liquid crystal cell such that the absorption axis of the one polarizer is aligned in parallel to the orientation direction of the liquid crystal molecules in the electric field-off state. Typically, this electric field-off state corresponds to a “black state”. On the other hand, a “white state” is achieved by applying an electric field to the liquid crystal cell to rotate the liquid crystal molecules horizontally in the plane of the cell to thereby cause phase retardation of light through the liquid crystal cell in such a manner that the light which has passed through the one polarizer is transmitted through the other polarizer. The “white state” is achieved when the liquid crystal molecules are oriented along axes each bisecting an intersecting angle between the absorption axes of the pair of polarizers, or, in other words, the liquid crystal molecules are oriented at an orientation angle of 45° with respect to the absorption axes of the polarizers, to provide a maximum light transmittance. In actual practice, however, it is difficult to have the liquid crystal molecules rotated to the position of the ideal orientation angle of 45°, and therefore an orientation angle substantially equal to, or close to, 45° is treated as being within the coverage of the “white state”. A liquid crystal display device is called as “O-mode” if the polarizer located at an illumination source-side has its absorption axis parallel to the orientation direction of the liquid crystal molecules in the electric field-off state, whereas, it is called as “E-mode” if the polarizer at a viewing-side has its absorption axis parallel to the orientation direction of the liquid crystal molecules in the electric field-off state.
In a liquid crystal display device equipped with the IPS-type liquid crystal cell, problems have been experienced in that, when a user observes a display screen at an angle of 45° with respect to the absorption axis of the viewing side polarizers and obliquely with respect to the plane of the liquid crystal cell, there is a decrease in contrast, and a change in display color is visually recognized when observed from different angles. In view of the problems, proposals have been made by, for example, the Patent Documents 1 and 2 which propose disposing a plurality of retardation films on one side of the liquid crystal cell to thereby solve the above problems. Further, as more effective solution than the proposals by the Patent Documents 1 and 2, the Patent Document 3 proposes to use two retardation films consisting of a “negative biaxial plate” and a “positive biaxial plate”, in combination with the one polarizer. According to this proposal, the negative biaxial plate is defined as an optical component having a relationship of nx1>ny1>nz1, where nx1 represents a refractive index in a slow axis (x-axis) direction, ny1 represents a refractive index in a fast axis (y-axis) direction, and nz1 represents a refractive index in a thickness-wise (z-axis) direction, whereas the positive biaxial plate is defined as an optical component having a relationship of nz2>nx2>ny2, where nx2 represents a refractive index in the slow axis (x-axis) direction, ny2 represents a refractive index in the fast axis (y-axis) direction, and nz2 represents a refractive index in the thickness-wise (z-axis) direction. Further, in the Patent Document 3, in-plane and thickness-wise retardation values of the negative and positive biaxial plates, and mutual relationships of the retardation values, are specified in detail. According to the teachings of the Patent Document 3, the arrangement proposed by this Document makes it possible to reduce light leakage in the oblique direction in a black state of a liquid crystal display device to enhance contract.